Since triangle fans and triangle strips do not support edge flags, the begin callback is not called with GLU_TRIANGLE_FAN or GLU_TRIANGLE_STRIP if a non-NULL edge flag callback is provided. (If the callback is initialized to NULL, there is no impact on performance). Instead, the fans and strips are converted to independent triangles. The function prototype for this callback is:

void edgeFlag( GLboolean flag );


                        

void combine( GLdouble coords[3], void *vertex_data[4], 


              GLfloat weight[4], void **outData );


                        

The vertex is defined as a linear combination of up to four existing vertices, stored in vertex_data. The coefficients of the linear combination are given by weight; these weights always add up to 1. All vertex pointers are valid even when some of the weights are 0. coords gives the location of the new vertex.

The user must allocate another vertex, interpolate parameters using vertex_data and weight, and return the new vertex pointer in outData. This handle is supplied during rendering callbacks. The user is responsible for freeing the memory some time after gluTessEndPolygon() is called.

For example, if the polygon lies in an arbitrary plane in 3-space, and a color is associated with each vertex, the GLU_TESS_COMBINE callback might look like this:

void myCombine( GLdouble coords[3], VERTEX *d[4],


                GLfloat w[4], VERTEX **dataOut )
{


   VERTEX *new = new_vertex();


   new->x = coords[0];


   new->y = coords[1];


   new->z = coords[2];


   new->r = w[0]*d[0]->r + w[1]*d[1]->r + w[2]*d[2]->r + w[3]*d[3]->r;


   new->g = w[0]*d[0]->g + w[1]*d[1]->g + w[2]*d[2]->g + w[3]*d[3]->g;


   new->b = w[0]*d[0]->b + w[1]*d[1]->b + w[2]*d[2]->b + w[3]*d[3]->b;


   new->a = w[0]*d[0]->a + w[1]*d[1]->a + w[2]*d[2]->a + w[3]*d[3]->a;


   *dataOut = new;
}


                        

If the tessellation detects an intersection, then the GLU_TESS_COMBINE or GLU_TESS_COMBINE_DATA callback (see below) must be defined, and it must write a non-NULL pointer into dataOut. Otherwise the GLU_TESS_NEED_COMBINE_CALLBACK error occurs, and no output is generated.

void error( GLenum errno );


                        

The GLU library will recover from the first four errors by inserting the missing call(s). GLU_TESS_COORD_TOO_LARGE indicates that some vertex coordinate exceeded the predefined constant GLU_TESS_MAX_COORD in absolute value, and that the value has been clamped. (Coordinate values must be small enough so that two can be multiplied together without overflow.) GLU_TESS_NEED_COMBINE_CALLBACK indicates that the tessellation detected an intersection between two edges in the input data, and the GLU_TESS_COMBINE or GLU_TESS_COMBINE_DATA callback was not provided. No output is generated. GLU_OUT_OF_MEMORY indicates that there is not enough memory so no output is generated.

gluTessCallback(tobj, GLU_TESS_BEGIN, glBegin);
gluTessCallback(tobj, GLU_TESS_VERTEX, glVertex3dv);
gluTessCallback(tobj, GLU_TESS_END, glEnd);
gluTessCallback(tobj, GLU_TESS_COMBINE, myCombine);
gluTessBeginPolygon(tobj, NULL);


   gluTessBeginContour(tobj);


      gluTessVertex(tobj, v, v);


      ...


   gluTessEndContour(tobj);
gluTessEndPolygon(tobj);